Method for the production of hydrocyanic acid by oxidation of nitrogen-containing hydrocarbons in a flame

ABSTRACT

A process is described for preparing hydrogen cyanide by autothermal noncatalytic oxidation of one or more nitrogenous hydrocarbons or a nitrogenous hydrocarbon mixture in which the nitrogenous hydrocarbons, an oxygen-containing gas, with or without ammonia, with or without water, with or without a gas containing nitrogen oxides and with or without other essentially inert feed gas constituents are introduced into a flame reaction zone, react in the flame reaction zone and a post-reaction zone at a temperature of from 1000 to 1800° C. for a reaction time of 0.03 to 0.3 s to form a cleavage gas which comprises at least the constituents hydrogen cyanide, carbon oxides, hydrogen, water, ammonia, nitrogen, light hydrocarbons with or without other cleavage gas constituents, the atomic C/N ratio in the reaction zones being from 1 to 7 and the atomic air ratio λ ato  being &lt;0.6, the cleavage gas being cooled and separated.

The invention relates to a process for preparing hydrogen cyanide byautothermal noncatalytic oxidation of nitrogenous hydrocarbons in aflame.

In the preparation of nitrogenous base chemicals such as acrylonitrile,adiponitrile or isocyanates, nitrogenous byproducts are sometimesproduced which cannot be further utilized materially since, for example,their isolation would be too complex or there is no economicallyefficient possibility for their direct material use. Such byproducts arefrequently produced as distillation residues, in the form of complexmixtures. Such residues have been disposed of to date, for example, bycombusting them, or by converting them in gasification processescompletely into combustible gas which is then thermally utilized, forexample in gas engines or gas turbines for generating electrical powerand heating energy.

M. Schingnitz and J. Görz describe in Umwelt, Volume 29 (1999), No. 9,the gasification of fuels, residues and wastes by entrained-flowgasification.

It is known to generate hydrogen cyanide in a flame by combustion ofmixtures of low aliphatic hydrocarbons, ammonia and oxygen. In addition,it is known to oxidize catalytically acetonitrile as a light nitrogenoushydrocarbon, hydrogen cyanide being obtained.

U.S. Pat. No. 2,596,421 discloses the flame synthesis of hydrogencyanide from ammonia, oxygen and light hydrocarbons having a maximum of6 carbon atoms. Three different laminar burners are proposed to carryout the flame synthesis.

DE-B 1 159 409 describes the preparation of hydrogen cyanide bynoncatalytic autothermal combustion of saturated or unsaturatedhydrocarbons having from 1 to 6 carbon atoms, ammonia and oxygen, andcooling the reacted gases immediately after exit from the combustionzone. The combustion takes place in a turbulent flame.

U.S. Pat. No. 4,981,670 describes the preparation of hydrogen cyanide bycatalytic oxidation of acetonitrile on an oxidation/amoxidation catalystin a fluidized-bed reactor.

It is an object of the present invention to provide a process for thematerial utilization of residues which comprise organically boundnitrogen.

We have found that this object is achieved by a process for preparinghydrogen cyanide by autothermal noncatalytic oxidation of one or morenitrogenous hydrocarbons or a nitrogenous hydrocarbon mixture in whichthe nitrogenous hydrocarbons, an oxygen-containing gas, with or withoutammonia, with or without water, with or without a gas containingnitrogen oxides and with or without other essentially inert feed gasconstituents are introduced into a flame reaction zone, react in theflame reaction zone and a post-reaction zone at a temperature of from1000 to 1800° C. for a reaction time of 0.03 to 0.3 s to form a cleavagegas which comprises at least the constituents hydrogen cyanide, carbonoxides, hydrogen, water, ammonia, nitrogen, light hydrocarbons with orwithout other cleavage gas constituents, the atomic C/N ratio in thereaction zones being from 1 to 7 and the atomic air ratio λ_(ato) being<0.6, the cleavage gas being cooled and separated.

The flame reaction zone is the reaction zone in which typicallyapproximately 95% fuel or oxidizer are reacted. The flame reaction zoneis distinguished by light emissions in the visible range and isoptically detectable as such. It is followed according to the inventionby a post-reaction zone.

The autothermal noncatalytic oxidation of the nitrogenous hydrocarbonscan be carried out in the presence or absence of ammonia additionallyintroduced into the flame. By adding ammonia, a defined C/N ratio can beset in the reaction mixture.

In order to achieve a satisfactory HCN yield, the atomic C/N ratio inthe flame should be from 1 to 7. If the atomic C/N ratio in thenitrogenous hydrocarbon or hydrocarbons used is >7, adding ammonia isgenerally necessary in order to decrease the C/N ratio to a value lessthan or equal to 7. If the atomic C/N ratio of the nitrogenoushydrocarbons used is in the range from 5 to 7, preferably ammonia isadded in order to decrease the C/N ratio further. If the atomic C/Nratio of the nitrogenous hydrocarbons used is in the range from 1 to 5,good HCN yields can be achieved without adding ammonia. Particularlypreferably, the atomic C/N ratio is in the range from 2 to 4, since thena high hydrogen cyanide yield is achieved with respect to the boundnitrogen and carbon in the fuel mixture and a high hydrogen cyanideconcentration in the cleavage gas is achieved.

The atomic C/N ratio is the ratio of the total bound carbon in the feedhydrocarbons to the total bound nitrogen in the feed hydrocarbons andwhere appropriate in the added ammonia and the added nitrogen oxides(NO, NO₂; N₂O is not taken into account) without taking into account themolecular nitrogen introduced with the oxygen-containing gas, forexample air. The carbon which is directly bound to oxygen, as inmethanol or formaldehyde, is not taken into account in the total carbonfor calculating the C/N ratio. Accordingly, in the case of ethanol, onlythe methyl carbon is taken into account.

Suitable nitrogenous feed hydrocarbons can be certain well-definedcompounds such as acetonitrile, propionitrile, adiponitrile ormethylglutaronitrile. Suitable nitrogenous feed hydrocarbons, however,can also be complex nitrogenous hydrocarbon mixtures. Such hydrocarbonmixtures are produced, for example, in the preparation of definednitrogenous compounds as residues, for example distillation residues.Examples are nitrogenous residues produced in the preparation ofadipodinitrile, acrylonitrile, aniline or isocyanates. Further examplesare refinery residues. The inventive process is suitable in particularfor supplying such residues to an economic material utilization.

In principle the inventive process is also suitable for materialutilization of nitrogen-free hydrocarbons or hydrocarbon mixtures whichare produced as residues. Such hydrocarbons are preferably low-oxygenhydrocarbons, for example contaminated solvents such as THF or hexane.

The oxygen-containing gas which is introduced into the flame andmaintains the combustion can be, for example, pure oxygen,oxygen-enriched air or air. Preferably, the oxygen-containing gas istechnical-grade oxygen. By using virtually pure oxygen, a hydrogencyanide concentration as high as possible in the cleavage gas isachieved, since large amounts of inert gases, such as molecularnitrogen, do not need to be introduced into the gas mixture. Thisfacilitates the workup of the cleavage gas and improves the thermalutilizability of the residual cleavage gas remaining after the removalof hydrogen cyanide.

For the hydrogen cyanide yield of the inventive process it is essentialthat the atomic air ratio λ_(ato) is <0.6. In addition, it is essentialfor the hydrogen cyanide yield that the temperature in the flame is inthe range from 1000 to 1800° C. It has been found that chemically boundnitrogen in the rich combustion mixtures, that is to say low-oxygencombustion mixtures, and at high temperatures principally reacts to formhydrogen cyanide. A temperature in the specified range and an atomic airratio λ_(ato)<0.6 lead, in addition, to light hydrocarbons such asmethane, ethylene and acetylene being present in the product gas mixtureexiting the flame reaction zone. The presence of these hydrocarbons inthe post-reaction zone counteracts the hydrogen cyanide breakdown in thepost-reaction zone.

The atomic air ratio is defined as follows:λ_(ato)=[O]/([H]/2+2×[C])

In the equation [O], [H] and [C] denote the total free or bound oxygen,hydrogen and carbon present in total in the combustion mixture.

The temperature in the flame can be controlled via the oxygen feed.Increasing the feed of molecular oxygen also causes an increase in flametemperature. The flame temperature can, in addition, be controlled byadding water. Adding water firstly causes cooling, but secondly alsoincreases the oxygen supply in the flame and thus the atomic air ratio.This counteracts unwanted soot formation.

In an embodiment of the inventive process, water is introduced into theflame. Water can be introduced into the flame in the vapor state or inliquid form, for example as hydrocarbon/water emulsion. A particularlygood cooling is achieved, because of the high evaporation enthalpy ofwater, when water is introduced in liquid form into the flame.

The temperature in the flame can also be controlled by adding gaseswhich substantially behave in an inert manner in the flame, which act asthermal ballast and thus cause cooling.

In a further embodiment of the inventive process, substantiallyinert-behaving further feed gas constituents are introduced into theflame. Such feed gas substituents are, for example, carbon oxides (COand CO₂), molecular hydrogen or oxygen. Essentially, inert-behaving feedgas constituents are those which behave substantially inertly withrespect to hydrogen cyanide formation, that is do not participate in thereactions leading to hydrogen cyanide formation.

The temperature in the flame is preferably from 1200 to 1400° C.

In addition to said gases, gases containing nitrogen oxides, that is tosay nitrogen monoxide or nitrogen dioxide, can be introduced into theflame. Nitrogen oxides are also converted into hydrogen cyanide withhigh yield in the inventive process. Thus the inventive process alsopermits the material utilization of exhaust gases containing nitrogenoxides, for example from nitric acid preparation.

In addition, it is essential for the hydrogen cyanide yield of theinventive process that the residence time of the reaction gas mixture isin the range from 0.03 to 0.3 s at the combustion temperature of from1000 to 1800° C. It has been found that a longer residence time and/orhigher temperatures leads to a significant breakdown of previouslyformed hydrogen cyanide. With a shorter residence time and/or lowertemperatures, the conversion rate achieved, in contrast, is inadequate.Both act in a manner to decrease the yield. A defined residence time insaid temperature range can be set by rapid cooling of the cleavage gasformed in the combustion.

The nitrogenous feed hydrocarbon or hydrocarbons can be introduced inliquid form or in the gaseous state into the flame.

The feed hydrocarbons can be introduced in liquid form into the flame.The hydrocarbons are preferably introduced in liquid form by atomizingthe hydrocarbons to fine liquid droplets, preferably having a meandroplet diameter of <100 μm, particularly preferably <50 λm, inparticular <20 μm. For the purposes of the present invention, meandroplet diameter is the Sauter diameter d₃₂ known to those skilled inthe art, which characterizes the specific surface area per unit volume.A droplet size as small as possible ensures that the time which theliquid droplets require for complete vaporization is small compared withthe reaction time, so that the vaporization takes placequasi-instantaneously. Typically, a 200 μm hydrocarbon droplet requiresapproximately 50 ms for complete vaporization, which is already in theorder of magnitude of the reaction time. The first vaporized hydrocarbonmolecules therefore find, even in the time mean of rich combustionconditions, a high oxygen supply, that is to say lean combustionconditions. However, lean combustion conditions decrease the hydrogencyanide yield. Large hydrocarbon droplets can even break through theflame.

The feed hydrocarbons can be atomized by means of a single-componentnozzle or, using an auxiliary atomizing medium, by means of atwo-component nozzle. Very fine atomization is frequently achieved usingan auxiliary atomizing medium. As auxiliary atomizing medium, theoxygen-containing gas used and/or other gas constituents of the reactiongas mixture to be introduced into the flame, such as ammonia, or steam,can be used.

It is also possible, provided that the additional feed of water, forexample for controlling the flame temperature and/or for increasing theoxygen supply, is provided, to introduce the nitrogenous hydrocarbon asan aqueous emulsion into the flame. Owing to the presence of water, there-atomization of the liquid droplets in the flame takes place. This iscaused by spontaneous vaporization of emulsified water droplets. Thismakes very fine atomization possible.

The nitrogenous feed hydrocarbon or hydrocarbons can be introduced inthe gaseous state into the flame. The nitrogenous hydrocarbons can bevaporized in advance, then premixed with further feed gas constituentsand introduced as gas mixture into the flame. Further feed gasconstituents with which the pre-vaporized hydrocarbons are mixed are, inparticular, the oxygen-containing gas with or without ammonia, but alsosteam and other inert-behaving gas constituents such as molecularhydrogen and carbon oxides. The feed hydrocarbon or feed hydrocarbonscan also be only partly pre-vaporized and/or premixed with only a partof the further feed gas constituents.

Preference is given to a burner construction which achieves flow as freefrom back-mixing as possible, since backmixing decreases the hydrogencyanide yield. Generally, in the flame reaction zone and thepost-reaction zone, turbulent flow prevails.

A cleavage gas is obtained which comprises at least the constituentshydrogen cyanide, carbon oxides, hydrogen, water, ammonia, nitrogen andlight hydrocarbons. Furthermore, the cleavage gas can comprise furtherconstituents such as nitrogen monoxide or isocyanic acid.

A typical cleavage gas composition comprises, as main constituents,hydrogen cyanide, ammonia, nitrogen, steam, carbon monoxide, carbondioxide and hydrogen and, as minor components, methane, ethylene andacetylene and, in traces, nitrogen monoxide and isocyanic acid.

Separation of a cleavage gas which comprises at least the constituentshydrogen cyanide, carbon oxides, hydrogen, water and ammonia, preferablycomprises the steps

-   -   (i) cooling the cleavage gas to a temperature<300° C.;    -   (ii) removing ammonia as ammonium sulfate or ammonium phosphate        by gas scrubbing, with an ammonia-depleted cleavage gas being        obtained;    -   (iii) removing hydrogen cyanide as aqueous hydrogen cyanide        solution, a hydrogen cyanide-depleted residual cleavage gas        being obtained;    -   (iv) recovering hydrogen cyanide from the aqueous hydrogen        cyanide solution by distillation;    -   (v) where appropriate, partially recirculating the residual        cleavage gas to the flame reaction zone.

The cleavage gas can be cooled in a plurality of stages. For example, ina first stage, by spraying water into the hot cleavage gas, a very rapidcooling to approximately 800° C. can take place. In a second stage, thehot cleavage gas can be cooled in a steam generator to approximately300° C. In a third stage, the cleavage gas can be cooled toapproximately 200° C. by spraying in oil. In this process, soot and sootprecursors (polycyclic hydrocarbons) formed during the combustion aresuspended or dissolved in the oil and are thus scrubbed out of thecleavage gas.

Ammonia can be removed by gas scrubbing using sulfuric acid, phosphoricacid or monoammonium phosphate solution, with ammonia being removed asammonium sulfate or ammonium phosphate. In this process anammonia-depleted cleavage gas is obtained; preferably an ammonia-freecleavage gas is obtained. If ammonia is removed as ammonium phosphate,ammonia can be stripped free from the aqueous solution using steam andrecycled to the flame. The recovered ammonium monophosphate solution canbe recirculated to the gas scrubber.

Hydrogen cyanide can be removed by condensing hydrogen cyanide and steamfrom the cleavage gas or by absorption in cold water, an aqueoushydrogen cyanide solution being obtained. Essentially anhydrous hydrogencyanide can be recovered from the aqueous hydrogen cyanide solution bydistillation.

A hydrogen cyanide-depleted residual cleavage gas, preferably anessentially hydrogen cyanide-free residual cleavage gas, remains which,for example, still comprises carbon oxides and molecular hydrogen and,in small amounts, minor constituents such as methane, ethylene andacetylene. The residual cleavage gas can be at least partiallyrecirculated into the flame as thermal ballast. The residual cleavagegas can, in addition, be thermally utilized.

The invention will be described in more detail below with reference tothe drawings.

FIG. 1 shows a process diagram of a preferred embodiment of theinventive process. The reactor designated overall by 1 comprises aburner 2 having a burner flame 2 a and feed lines 3 to 8. Via feedlines3 and 4, natural gas and air are introduced to operate a pilot burnerand to heat the reactor. Via feedlines 5, 6, 7 and 8, nitrogenous liquidresidue, oxygen, ammonia and steam are introduced. Via feedline 9 andthe outlet line 9 a, the reactor is supplied with cooling water. Theresultant hot cleavage gases 10 are rapidly cooled to approximately 800°C. by spraying in water 11 into the hot cleavage gas in a line 12 havingan expanded pipe cross-section. The cleavage gas thus precooled ispassed to a steam generator 13 having a water feedline 14 and steamoutlet line 15 and is there cooled to approximately 300° C. In theVenturi scrubber 16, the cleavage gas is finally cooled to approximately200° C. by spraying in quenching oil 17, with soot-containing oil 18being produced. To remove ammonia, the cooled cleavage gas 19 isscrubbed in a separation column 20 using sulfuric acid, phosphoric acidor monoammonium phosphate solution 20 a, with ammonia being removed asammonium sulfate or ammonium phosphate. At the top of the column anammonia-free cleavage gas 21 is obtained, and at the bottom of thecolumn an aqueous solution 22 of ammonium phosphate or ammonium sulfateis obtained. To remove hydrogen cyanide, the ammonia-free cleavage gas21 is passed to a further separation column 23 in which hydrogen cyanideis absorbed from the cleavage gas in cold water 24. At the bottom of thecolumn an aqueous hydrogen cyanide solution 25 is obtained, at the topof the column an essentially hydrogen cyanide-free residual cleavage gas26 is obtained which can be used, for example, as synthesis gas. Theaqueous hydrogen cyanide solution 25 is passed to a further separationcolumn 27 and there separated into essentially anhydrous hydrogencyanide 28 as overhead product and water 29 as bottom product. Foremergencies, and for startup and shutdown operations, in addition, aconventional flare 30 is provided.

A hydrogen cyanide-depleted residual cleavage gas 26, preferably anessentially hydrogen cyanide-free residual cleavage gas 26, remains,which still comprises, for example, carbon oxides and molecular hydrogenand, in small amounts, minor components such as methane, ethylene andacetylene. The residual cleavage gas can be at least in partrecirculated into the flame as thermal ballast. The residual cleavagegas can, in addition, be thermally utilized.

FIG. 2 a shows a section through a burner used in the inventive processfor generating a flame reaction zone. The burner, which is designatedoverall 39, and is anchored in a combustion chamber lid 40, comprises,incorporated into a burner brick 41, a pilot burner 31 having an airfeedline 32 and a natural gas feedline 33 having corresponding orifices32 a and 33 a, respectively, a multiplicity of gas feedlines 34 forinert gases and gas feedlines 35 for ammonia concentrically arranged inthese having corresponding orifices 34 a and 35 a, respectively, and amultiplicity of two-component nozzles 36 having feedlines 37 for oxygenand feedlines 38 for nitrogenous liquid residues concentrically arrangedin these having corresponding orifices 37 a and annular gap orifices 38a, respectively.

FIG. 2 b shows a plan view of the burner together with the orifices 32 ato 38 a.

FIG. 3 shows a section through a reactor used in the inventive process.Connected to the burner 39 which is incorporated into the burner lid 40(shown simplified) is the cylindrical combustion chamber 42 of diameterD and length L, which is bound by the high-temperature insulation 43(for example of Al₂O₃ and SiO₂). The high-temperature insulation 43 issurrounded by the rear insulation 44 and the steel shell 45. This isconnected via a flange 46 and seals 47 to the burner lid 40. Thegeometry of the combustion chamber achieves a low-backmixing plug flow.

EXAMPLE

In a laboratory system, a study is made of the autothermal noncatalyticpartial oxidation of acetonitrile which serves as simple model substancefor highly nitrogeneous hydrocarbons. The laboratory system comprises aburner having an outer diameter of 15 mm which has six axial 4 mmboreholes for feeding in the premixed starting materials and a central 2mm borehole for feeding in oxygen. A 1.1 m long tube made of aluminumoxide having an internal diameter of 15 mm is mounted on the burner,which tube is situated in an electrically heated furnace. The furnacetemperature is selected to be equal to the calculated reactiontemperature of 1300° C., to minimize heat losses. 1.93 kg/h ofacetonitrile and 0.19 kg/h of water are metered in two separatethin-film evaporators and vaporized. The vapors are homogeneously mixedwith 0.12 m³ (S.T.P.)/h of oxygen and fed through the six axialboreholes at 120° C. Through the central borehole is fed 0.12 m³(S.T.P.)/h of oxygen to stabilize the flame. The residence time of thereaction gases at the reaction temperature of 1300° C. is calculated tobe 0.05 s. The cleavage gas formed is then rapidly cooled to about 150°C. by water-cooled walls and the gas composition is thus “frozen”. Thecooler has a 90° bend having an internal diameter of 29.7 mm, to whichis attached a 485 mm long straight piece having an internal diameter of20 mm. The cooled length between the end of the reactor and the samplingpoint is in total 670 mm. The composition of the cleavage gas mixture isthen determined. HCN and NH₃ are determined using an FT-IR spectrometer,in which case for determining HCN, the cleavage gas is dried in advanceover P₂O₅, whereas NH₃ is determined in the gas mixture which containswater vapor. CO, CO₂ and CH₄ are determined by ND-IR, CH₄, C₂H₂, C₂H₄and C₂H₆ are determined by GC-FID and H₂ is determined using a thermalconductivity detector, with HCN, NH₃ and H₂O being removed in advance byNaOH or P₂O₅.

The composition below was determined (in % by volume, based on theanhydrous gas mixture): HCN 29 NH₃ 1.8 CO 28 CO₂ 1.4 H₂ 26 CH₄ 1.8 C₂H₂2 C₂H₄ 0.1 C₂H₆ 0.001

This gives an HCN yield of approximately 70%, based on nitrogen used.

1. A process for preparing hydrogen cyanide by autothermal noncatalyticoxidation of one or more nitrogenous hydrocarbons or a nitrogenoushydrocarbon mixture wherein said nitrogenous hydrocarbons, anoxygen-containing gas, with or without ammonia, with or without water,with or without a gas comprising nitrogen oxides and with or withoutother essentially inert feed gas constituents are introduced into aflame reaction zone, react in the flame reaction zone and apost-reaction zone at a temperature of from 1000 to 1800° C. for areaction time of 0.03 to 0.3 s to form a cleavage gas which compriseshydrogen cyanide, carbon oxides, hydrogen, water, ammonia, nitrogen,light hydrocarbons with or without other cleavage gas constituents, theatomic C/N ratio in the reaction zones being from 1 to 7 and the atomicair ratio λ_(ato) being <0.6, said cleavage gas being cooled andseparated.
 2. A process as claimed in claim 1, wherein said ammonia isintroduced into said flame reaction zone.
 3. A process as claimed inclaim 1, wherein said nitrogenous hydrocarbons have a C/N ratio of from1 to
 5. 4. A process as claimed in claim 1, wherein said nitrogenoushydrocarbons have a C/N ratio of from 5 to
 7. 5. A process as claimed inclaim 1, wherein said nitrogenous hydrocarbons are one or more residues.6. A process as claimed in claim 1, wherein water is introduced intosaid flame reaction zone.
 7. A process as claimed in claim 1, whereinone or more additional essentially inert-behaving feed gas constituentsare introduced into said flame reaction zone.
 8. A process as claimed inclaim 7, wherein said essentially inert-behaving feed gas constituentscomprise carbon oxides and/or hydrogen obtained from said cleavage gas.9. A process as claimed in claim 1, wherein said gas comprising nitrogenoxide is introduced into said flame reaction zone.
 10. A process asclaimed in claim 1 wherein said nitrogenous hydrocarbons or saidhydrocarbon mixture are/is introduced in liquid form into said flamereaction zone.
 11. A process as claimed in claim 10, wherein saidnitrogenous hydrocarbons are atomized to form liquid droplets having amean particle diameter of <100 μm.
 12. A process as claimed in claim 10,wherein said nitrogenous hydrocarbons or said hydrocarbon mixture is/areintroduced as an aqueous emulsion into said flame reaction zone.
 13. Aprocess as claimed in claim 12, wherein said aqueous emulsion isatomized to form liquid droplets having a particle diameter of <100 μm.14. A process as claimed in claim 1, wherein said nitrogenoushydrocarbons are introduced in the gaseous state into said flamereaction zone.
 15. A process as claimed in claim 14, wherein saidgaseous nitrogenous hydrocarbons are premixed with at least a part ofthe feed gas constituents selected from the group consisting of saidoxygen-containing gas, said ammonia, said gas comprising nitrogen oxide,said water and said essentially inert-behaving feed gas constituents,and the resultant gas mixture is introduced into said flame reactionzone.
 16. A process as claimed in claim 1, wherein turbulent flowprevails in said reaction zones.
 17. A process as claimed in claim 1,wherein the separation of said cleavage gas comprises: (i) cooling saidcleavage gas to a temperature<300° C.; (ii) removing said ammonia asammonium sulfate or ammonium phosphate by gas scrubbing to obtain anammonia-depleted cleavage gas; (iii) removing said hydrogen cyanide asaqueous hydrogen cyanide solution to obtain a hydrogen cyanide-depletedand ammonia-depleted residual cleavage gas; (iv) recovering hydrogencyanide from said aqueous hydrogen cyanide solution by distillation; and(v) where appropriate, partially recirculating said residual cleavagegas to said flame reaction zone.